The nature of the cell membrane receptor to which glucose and other monosaccharides is bound prior to and during their penetration into the cell is being studied. Phlorizin and a number or organically synthesized compounds related in structure to phlorizin have been found to be preferentially boudn to this glucose transport receptor and thus inhibit the membrane penetration of sugars. This characteristic binding property serves as a fingerprint which can be used to identify the membrane component in attempts to isolate it from the cell membranes of a number of mammalian tissues. Radioactively labelled phlorizin and phlorizin analogs have been prepared to serve as easily measurable marker molecules in these investigations. Procedures are being developed whereby cell membranes are being solubilized and fractionated by affinity chromatography and other conventional methods. Attempts are being made to learn more about the ways in which membrane enzymes and other components are constructed in this cell barrier.